The present invention relates to an electric capacitor with an enclosure structure consisting of a laminated film and, more particularly, to an electric capacitor with an enclosure structure which is obtained by coating a capacitor element with a thermoplastic laminated film and sealing the outer surface of the obtained structure by heat sealing.
Various types of capacitors such as aluminum electrolytic capacitors, tantalum electrolytic capacitors, plastic film capacitors, paper capacitors, and ceramic capacitors are obtained by enclosing capacitor elements in capacitor enclosures made of a metal such as aluminum, brass, tantalum, silver or nickel, or a synthetic resin such as phenol resin, polypropylene resin, polyethylene resin polyphenylene sulfide resin, or polysulfone resin; or by coating by molding such capacitor elements with a synthetic resin such as epoxy resin or polyester resin. However, with the former type of capacitor which is obtained by enclosing a capacitor element in an enclosure of a metal or a synthetic resin, a relatively great gap is formed between the capacitor enclosure and the capacitor element, so that the overall shape of the capacitor enclosure becomes bulky. Furthermore, the procedure for enclosing the capacitor element in the capacitor enclosure is complex and is time-consuming. On the other hand, with the latter type of capacitor which is obtained by coating the capacitor element with a synthetic resin, control of the thickness of the coating of the synthetic resin is difficult, resulting in non-uniform thickness. Moreover, fine undulations are formed on the surface of the coating, resulting in poor outer appearance of the capacitor.
A capacitor with another enclosure structure is also known in which the structure is obtained by preparing two thermosetting resin sheets each having a recess to accommodate a capacitor element, placing the capacitor element in a cavity formed by opposing the recesses of the two thermosetting resin sheets, adhering the thermosetting resin sheets, and finally pressing under heating to form the structure. However, this type of capacitor is subjected to pressing under heating for a predetermined interval during formation of the enclosure structure from the thermosetting resin sheets. Accordingly, the capacitor element is subjected to thermal stress, and evaporation and/or deterioration of an electrolyte or impregnating agent, and degradation of the capacitor characteristics may be caused.
A capacitor with still another enclosure structure is known in which the enclosure structure is obtained by wrapping a capacitor element by a laminated film of two plastic films with or without a metal foil layer interposed therebetween, and forming a sealed portion of 0.5 to 0.7 mm width at the periphery of the laminated film by heat sealing.
However, it has been found that the width of the sealed portion of the laminated film largely influences the performance of the capacitor with the enclosure structure of this type. More specifically, since the width of the sealed portion is 0.5 to 0.7 mm in the capacitor of this type, operation of the capacitor at a high temperature of, for example, 85.degree. C. for 1,000 hours or longer results in an increase in the internal pressure within the capacitor enclosure. At some point, the sealed portion becomes unable to withstand the internal pressure, causing leakage of the electrolyte and/or impregnating agent of the capacitor therethrough, or causing separation of the sealed portion and failing to sustain the hermetic seal. This may adversely affect the electrostatic capacitance or the dissipation factor tan .delta. to shorten the life time of the capacitor. However, it has been found that the hermetic seal of the enclosure of the capacitor is significantly improved and the life time of the capacitor is similarly improved if the width of the sealed portion is 1 mm or more.
It has also been found that the thickness of the inner film of the laminated film opposing the capacitor element has a great influence on the characteristics of the capacitor having the enclosure structure of the type as described above. In the conventional capacitors of this type, the thickness of the inner film is, for example, 5 to 50 .mu.m and 350 to 500 .mu.m. With the capacitors of the type wherein the thickness of the inner film is 5 to 50 .mu.m, operation at a high temperature for a long period of time tends to cause separation of the sealed portion or leakage of the electrolyte and/or impregnating agent. Particularly, if the laminated film of the plastic films with the metal foil layer interposed therebetween is used, short-circuiting tends to be caused between the external connecting terminals. On the other hand, with the capacitors of the type wherein the thickness of the inner film is 350 to 500 .mu.m, the electrolyte may evaporate through the section of the inner film or the cleaning agent may be introduced therethrough to shorten the life time of the capacitor. However, it has been found that if the thickness of the inner film is kept within the range of 70 to 200 .mu.m, these problems of separation of the sealed portion, leakage of the electrolyte and/or impregnating agent, short-circuiting between the external connecting terminals, and evaporation of the electrolyte or introduction of the cleaning agent through the section of the inner film may be effectively prevented, whereby the life time of the capacitor is significantly improved.
It has also been found that, in the capacitor having the capacitor enclosure structure consisting of a laminated film, the type of material of the laminated film largely influences the characteristics of the capacitor. Polyethylene or a copolymer mainly consisting of polyethylene is known as the material of the laminated film used for the conventional capacitor of this type. Although these materials are excellent in adhesion strength, flex resistance and oil resistance, they have poor resistance to solvents such as alcohols or ketones. However, if the laminated film consists of at least two different films each consisting of polyethylene terephthalate, polypropylene, polycarbonate, polysulfone, ionomer, polyvinylidene fluoride, polyethylene fluoride, polyvinylidene chloride, or polybutylene terephthalate, a capacitor is obtained which has an excellent resistance to solvents.